Clays are mined from the earth in a wet condition, containing about 15% to about 35% by weight water, and must be dried and crushed to a desired particle size distribution before being useful in essentially any of the industries in which clays are used. The milling or crushing process of the present invention is useful for any clay that requires a reduction in particle size to make that clay useful for a particular purpose. While the process of the present invention is particularly useful for crushing a smectite clay, it is also useful for kaolin clay; serpentines; talc and pyrophyllite; illite; glauconite; chlorite and vermiculite; palygorslite and sepiolite; allophane and imogolite; diaspore clay; boehmite; and mixtures thereof. The preferred smectite clays milled in accordance with the principles of the present invention include montmorillonite; beidellite; nontronite; hectorite; saponite; sauconite; and mixtures thereof.
In the milling or crushing of any of the above-mentioned clays to prepare the clay for an industrial use, it is desirable to minimize the amount of very fine or powdery clay particles produced in the crushing process. Very fine, powdery clay particles are undesirable due to their dusting characteristics, presenting environmental problems in the plant and, for most industrial uses, the fine, powdery clay particles are not useful. Prior art milling processes for grinding clay to a desired particle size, such as a clay particle size distribution between about 250 microns and 707 microns (plus 25, minus 60 mesh, U.S. Sieve Series) result in ground particles that include about 30% by weight fines or powder (having a particle size less than about 250 microns) that must be discarded or otherwise processed, such as by pelletizing or otherwise granulating the fine particles, so that they can be reground to a useful particle size. Extant milling processes for grinding clay to a desired particle size distribution achieve only about a 70% yield (30% of the clay feed to the milling process is ground into fines or powder having a size less than about 250 microns and must be further treated to increase the particle size to make this portion of the clay useful).
In accordance with the principles of the present invention, an apparatus and method for milling or crushing clay has been discovered that surprisingly provides a yield of about 85% to about 95% by weight, usually about 90% to about 95% by weight yield, so that only about 5% to about 15% by weight of the clay feed to the apparatus, usually about 5% to about 10% by weight, need be discarded or granulated.